Acoustic investigation techniques are widely used as non-destructive tools to check the integrity of wells and their casing. As an example, noise logging has been used for almost 30 years to detect the location of gas leaks behind a casing of a well.
Gas leaks behind casing generally occur when a gas-bearing zone has not been properly isolated during the well construction process. The lack of zonal isolation allows gas to flow from the gas-bearing zone to the surface or to another subterranean zone outside the casing. The gas leak may for example cause an uncontrolled accumulation of gas behind the casing or at the surface of the well, and lead to a hazardous situations such as the contamination of a water table surrounding the well or the creation of an explosive mixture of gas at the surface.
Turning now to FIG. 1, a schematic diagram illustrating a general principle of the logging operation in a well is shown. A tool or sonde 10, for acquiring noise data is located in a borehole 11 penetrating an earth formation 12. The borehole 11 is lined by a casing 13. The sonde 10 is lowered in the borehole 11 by a cable 14 and slowly raised by a surface equipment 15 over a sheave wheel 16 while noise data is recorded. A depth of the sonde 10 is measured using a depth gauge 17 which measures cable displacement. Noise data acquired by the sonde 10 may be analysed either in situ near the sonde 10, or analysed by a data processor 18 at the surface, or stored, for later analysis.
Reliable detection of the position of a leak in the borehole is critical in designing a repair job for that leak and for subsequent determination of the success of leak repair.
Many techniques have been used to detect a position of a fluid leak in a borehole or in other environments. These techniques have been applied either individually or in combination with each other as will be understood from the prior art documents described hereafter.
One form of noise logging tool was proposed in the 1970's and is described in more detail in the following references:                McKinley, R. M., Bower, F. M., and Rumble, R. C.: “The Structure and Interpretation of Noise From Flow Behind Cemented Casing,” SPE3999, JPT, March 1973, P329,        Robinson, W. S.: “Field Results From the Noise-Logging Technique,” SPE5088, JPT, November 1976, P1370,        McKinley, and R. M., Bower, F. M.: “Specialized Applications of Noise Logging,” SPE6784, JPT, March 1979, P1387,        Brift, E. L.: “Theory and Applications of the Borehole Audio Tracer Survey,” SPE6552, Transactions of the SPWLA Seventeenth Annual Logging Symposium, Jun. 9-12, 1976, Denver, Colo.,        Koerner, Jr., H. B., and Carroll, J. C.: “Use of the Noise Log as a Downhole Diagnostic Tool,” SPE7774, presented at the SPE Middle East Oil Technical Conference, Bahrain, 25-29 Mar. 1979.        
The tool generally contains 4-6 high pass filters that transmit noise amplitudes above 200, 600, 1000, 2000, 4000 and 6000 Hz. For tools with the lowest number of filters, the 4000 and 6000 Hz cutoffs are eliminated. The noise data provided at surface is an average of these measured transmitted noise amplitudes over a certain time period, 10 seconds for example. The coarse frequency resolution and the time averaging limit the application of this type of tool to relatively high leak rates where the noise generated is semi-continuous and significant compared to background noise.